The panel’s decision removed a major obstacle that for more than two months had hindered the Justice Department’s investigation into Mr. Trump’s handling of sensitive government documents.
Both sides presented closing arguments in the trial of Stewart Rhodes and four other members of the far-right militia. They have been charged with seditious conspiracy for their roles in the Jan. 6 Capitol attack.
Mr. Smith will take on the role of special counsel after leaving his current position as a specialist prosecutor based in The Hague investigating war crimes.
It was the latest salvo in a court fight over a special master, which has stalled an investigation into the former president’s hoarding of government documents.
The ruling would effectively bar federal prosecutors from using a key piece of evidence as they continue to investigate whether Mr. Trump illegally retained national defense documents at his estate.